1. Field of the Invention
The present invention relates generally to the production of polyolefins and, more specifically, to the reduction and/or prevention of fouling in polyolefin reactors.
2. Description of the Related Art
This section is intended to introduce the reader to aspects of art that may be related to aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
As chemical and petrochemical technologies have advanced, the products of these technologies have become increasingly prevalent in society. In particular, as techniques for bonding simple molecular building blocks into longer chains (or polymers) have advanced, the polymer products, typically in the form of various plastics, have been increasingly incorporated into various everyday items. For example, polyolefin polymers, such as polyethylene, polypropylene, and their copolymers, are used for retail and pharmaceutical packaging, food and beverage packaging (such as juice and soda bottles), household containers (such as pails and boxes), household items (such as appliances, furniture, carpeting, and toys), automobile components, pipes, conduits, and various other consumer and industrial products.
One benefit of polyolefin construction, as may be deduced from the list of uses above, is that it is generally non-reactive with goods or products with which it is in contact as well as with the ambient environment. This property allows polyolefin products to be used in many residential, commercial, and industrial contexts, including food and beverage storage and transportation, consumer electronics, agriculture, shipping, and vehicular construction. The wide variety of residential, commercial and industrial uses for polyolefins has translated into a substantial demand for raw polyolefin which can be extruded, injected, blown or otherwise formed into a final consumable product or component.
The raw polyolefin is typically produced in bulk by petrochemical facilities, which have ready access to monomers, such as ethylene, that serve as the molecular building blocks of the polyolefins to be produced. The polymerization reaction itself is exothermic, or heat-generating, and is typically performed in closed systems where temperature and pressure can be regulated to produce polyolefins having the desired properties.
However, in some circumstances a polyolefin reactor may foul, such as when the polymerized product is formed on the reactor walls or when the product cannot be maintained as a slurry. Such a foul may result in a loss in heat transfer, such as due to a reduction in circulation or reduced efficiency at a heat exchanger interface, which may impair or completely negate the capacity to maintain the desired temperature within the reactor. A reactor foul may also result in a reduction in the circulation of the reactor contents and/or in a variation from the desired percent solids (measured by volume or by weight) of the reactor effluent To the extent that a reactor foul may result in deviations from the desired reaction conditions, the polymer product produced during such a reactor foul may not meet the desired specifications; that is, the product may be “off-spec.” In extreme or runaway fouling situations, control of the reaction may be lost entirely, and the reactor may become plugged with polymer, requiring one to three weeks to clear, during which time the reactor may not be operated.
A reactor foul may occur due to a variety of different factors, depending on the type of polymerization system and circumstances. Depending on the type of reactor foul, the external indications that such a foul exists may include deviations from the set reaction temperature or increased demand on the coolant system to maintain the set temperature value. Similarly, an increase in the temperature differential between the coolant inlet temperature and reactor temperature may be indicative of certain types of reactor fouls, such as those which interfere with the transfer of heat through the reactor walls. Another external indication of a foul may be an increased motor load as the pump attempts to maintain a velocity within the reactor sufficient to keep the polymer and catalyst particles suspended or attempts to compensate for restriction or obstruction of the flow path. Similarly, a high pressure differential may be observed at the pump and may indicate the presence of some fouls.
For example, copolymer fouling may occur when the reactor temperature falls above the “fouling curve,” describing the suitable reactor temperature ranges for producing polyolefins having a desired density. Such a deviation may result in swelling of the polymer particles and an increased tendency for the particles to agglomerate into larger particles, both which can increase the polymer volume in the reactor. The higher volume percent solids may result in a moving bed of polymer rather than a slurry, which decreases the circulation rate. To compensate, the reactor circulating pump must work harder to propel the fluid and particles, resulting in a high motor load and a high pressure differential, i.e., ΔP.
Similarly, a condition known as a “solids foul” may occur in which circulation of the reactants and product in the reactor is interrupted or degraded. For example, when reactor solids and ethylene concentrations are too high, large polymer particles may be formed which can plug continuous take-off valves or other outlet valves or conduits. The large polymer particles may also settle out of the slurry in the reactor, where they may restrict the flow of slurry. Furthermore, the large polymer particles increase volume percent solids in the reactor, increasing the flow resistance of the slurry and leading to a corresponding high motor load and a high ΔP as the reactor circulating pump compensates for the increased resistance.
An increase in fine particles of polymer, i.e., “fines,” may also result in a form of fouling. In particular, an increased number of fines increases the viscosity of the slurry due to the corresponding increase in particulate surface area. To compensate for the increase in viscosity, the reactor circulating pump must work harder, resulting in a higher motor load and ΔP. In these situations, if the pump is unable to compensate, heat transfer through the reactor walls may be impaired and/or polymer particles may settle out of the slurry.
Another type of fouling that may occur, depending on the reaction environment, is static fouling. Static fouling is typically associated with fines and/or catalysts being held to the reactor wall by static electricity. The catalyst particles and catalyst within the fines facilitate polymerization along the reactor wall, resulting in a film or layer of polymer growing on the reactor wall. As the layer of polymer grows, it decreases the transfer of heat from the reactor to the reactor coolant. The loss of heat transfer resulting from the polymer layer may result in a lowering of the coolant temperature at the inlet to maintain the desired production rate. As a result, the temperature differential, i.e., the spread, between the coolant inlet temperature and reactor temperature may increase. Furthermore, the layer of polymer restricts the flow of slurry along the reactor wall, resulting in an increased motor load and ΔP at the circulating pump.
As might be expected, a reactor foul may be indicated by some or all of the factors mentioned above. For example, a decreasing heat transfer rate, an increasing temperature differential, an increased motor load, and/or an increased ΔP may indicate the presence or development of a reactor foul. In response to these indicators, a rapid response is typically required to regain control of the reaction. Depending on the foul, such responses may include adjusting the reactor temperature, increasing the addition rate of diluent (such as isobutane), decreasing the addition rate of monomer, adding anti-static agents, and/or decreasing the addition rate of catalyst. If control of the reaction cannot be regained, it may be necessary to kill or moderate the reaction to prevent the reactor from becoming plugged with polymer.
In view of the limited response time which may be provided by the available fouling indicators, it may be desirable to prevent fouls from developing. Alternatively, to the extent fouls cannot be eliminated, it may be desirable to provide more warning of an impending foul so that less drastic responses may be employed to address the foul.